"This report briefly describes current responsibilities and selection mechanisms for 15 House and Senate party leadership posts and provides tables with historical data, including service dates, party affiliation, and other information for each. Tables have been updated as of the report's issuance date to reflect leadership changes. Although party divisions appeared almost from the First Congress, the formally structured party leadership organizations now taken for granted are a relatively modern development. Constitutionally specified leaders, namely the Speaker of the House and the President pro tempore of the Senate, can be identified since the first Congress. Other leadership posts, however, were not formally recognized until about the middle of the 19th century, and some are 20th century creations. In the earliest Congresses, those House Members who took some role in leading their party were often designated by the President as his spokesperson in the chamber. By the early 1800s, an informal system developed when the Speaker began naming his lieutenant to chair one of the most influential House committees. Eventually, other members wielded significant influence via other committee posts (e.g., the post-1880 Committee on Rules). By the end of the 19th century, the formal position of floor leaders had been established in the House."
CRS Report for Congress, RL30567